Cable networks originally carried programming from a head end to subscribers over a network of coaxial cable. Over time, these networks have changed. Some cable networks now include fiber optic links as part of the network. This variety of cable network is colloquially referred to as a “hybrid fiber/coax” (HFC) network.
A hybrid fiber/coax network typically includes a head end that broadcasts programming over the network to subscribers in a downstream direction. The network includes two main portions. The first portion of the network is optical links that connect the head end with a number of geographically dispersed distribution nodes. These nodes are referred to as “optical distribution nodes” or “ODNs.” At the ODNs, signals from the head end that carry the programming are converted from optical signals to electrical signals. The second portion of the network is coaxial links that connect the ODNs with subscriber equipment. The electrical signals are transmitted to the subscriber equipment over the coaxial cable links.
In recent years, the cable industry has experimented with systems that allow for bi-directional communication between subscriber equipment and the head end. This allows for services such as video-on-demand, telephony and Internet traffic to be offered over a cable network. Typically the 5 to 42 MHZ frequency range is reserved for upstream transmission from customers to the head end. Frequencies between 50 MHZ and an upper limit, e.g., 750 MHZ or 850 MHZ, typically carry downstream transmissions.
The design of the reverse path for transporting data over a hybrid fiber/coax network is laced with difficult technical issues. First, many customers must communicate over a common coaxial cable. Interference between customers and noise ingress onto the cable can cause disruptions and errors in this communication. Ingress and other interference is especially a problem at the low frequencies typically prescribed for upstream communications. Transporting simultaneous data transmissions from many customers also introduces complexity into the system design.
In most current systems, the reverse path is implemented with one of a number of different analog modulation schemes, e.g., MCNS, Data Over Cable Service Interface Specification (DOCSIS). These schemes are complicated to implement due to strict timing requirements and complex modulation schemes. Other systems, such as AT&T's mini fiber node (mFNs), introduce other complexities into the return path.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved digital data path for a hybrid fiber/coax network.